Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The annotations already declare readOnlyHint=true, so the safety profile is covered structurally. The description adds return-content context ('with its unit prices per currency') and the underlying endpoint, which is modestly useful, but says nothing about auth requirements or error behavior for an unknown id. With annotations carrying the safety story, this is adequate but not rich.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Two short sentences with no filler, and the substantive content (single product plus per-currency prices) is front-loaded ahead of the endpoint notation. The endpoint string is arguably redundant for an MCP tool but costs little.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a one-parameter read with no output schema, describing the return as a product with per-currency unit prices gives the agent enough to know what comes back. Nothing critical is missing, though it could mention what happens if the id is not found.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

There is a single parameter and schema description coverage is 100%, so the schema fully documents 'id' including its pattern and provenance. The description only restates 'by id' and adds the per-currency output notion, which is not parameter syntax. Baseline 3 applies when the schema does the heavy lifting.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

States a specific verb and resource ('One product by id with its unit prices per currency'), and the phrase 'one product by id' implicitly distinguishes it from the sibling billy_list_products. It stops short of explicitly naming the list counterpart as the alternative, so it is clear but not fully sibling-differentiated.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

No when-to-use, when-not-to-use, or alternative guidance appears in the description. The only routing hint ('from billy_list_products') lives in the schema, not the description, so the agent gets no explicit signal about when to call this versus list_products.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.